1. Field of the Invention
The present invention relates to an optical head apparatus for recording, reproducing or erasing information to and from an optical medium such as an optical disk or an optical card.
2. Description of the Prior Art
Optical memory technology uses an optical disk having a bit pattern as a high density, high capacity storage medium, and it has been expanding its practical uses which include digital audio disks, video disks, document file disks and data files.
A mechanism which records or reproduces information to and from an optical disk with a light beam converted to a micro-spot with high reliability depends mainly on its optical system. The mechanism has an optical head apparatus as a main component. Basic functions of the optical head apparatus are divided mainly into converging a light beam to form a micro-spot of diffraction limit, focusing and tracking control of the optical system, and detection of pit signals. These are realized by combining various optical system with various photoelectric conversion system according to object and an use of the optical head apparatus.
Recently, because a design of an optical system is improved and a diode laser as a light source which generates a light of shorter wavelength is fabricated, an optical disk with a higher density having a higher storage capacity has been developed. An approach for higher density is increase in numerical aperture (NA) on an optical disk of a optical system which converges a light beam to a micro-spot on an optical disk.
An increase in abberation due to inclination (so-called tilt) of an optical disk is a problem in this approach. That is, if numerical aperture is increased, abberation due to tilt is increased. This problem is prevented by decreasing the thickness of a substrate of an optical disk. For example, in order to have the same tolerance limit of abberation for an optical disk of NA=0.5 and thickness of 1.2 mm, the thickness is decreased to 0.6 mm for NA=0.6.
Then, it is desirable for a high density optical disk to have a smaller thickness. Thus, the thickness of a substrate of a higher density optical disk of next generation such as a digital versatile disk will be smaller than that of many prior art optical disks such as a compact disk (CD).
Therefore, it is desired to produce an optical disk drive which can record and reproduce both a prior art optical disk and a higher density optical disk of the next generation. Then, an optical head apparatus is necessary which converges a light beam of diffraction limit on optical disks having different thicknesses of substrate.
The inventors already proposed a light head which can be used with optical disks having different thicknesses of substrate (Japanese Patent Application No. 5-318230/1993) In the optical head, a light beam from a light source is collimated, and the beam is converged by a complex lens to a micro-spot on an optical disk. The light reflected by the optical disk returns the same optical path. Then, it is reflected by a beam splitter to be detected by a photosensor. A 2-focus lens comprising an object lens and a hologram lens which diffracts a part of the incident light is used for the optical head to form a converging spot of diffraction limit on optical disks having different thicknesses. The hologram lens has, for example, a concentric grating pattern in order to diffract a part of the incident beam, and a transmission light (or zeroth order diffraction light) also has a sufficient intensity. The light diffracted by the hologram lens and the other light not diffracted thereby are converged on different focus positions on an optical axis. Thus, a micro-spot can be formed on substrates of different thicknesses. Because the hologram lens functions as a lens, the positions of the two focuses are different on the optical axis from each other. When information is recorded or reproduced at one focus, the light beam converging on the other focus spreads largely to have a small optical intensity and it does not affect the recording or reproduction.
In the above-mentioned optical head apparatus using the two-focus lens has points to be improved or developed. For example, the 2-focus lens comprising an object lens and a hologram lens may have different structures.
In order to provide a compact optical head apparatus, a laser diode is desirable as the light source. However, the laser diode has a problem to be solved by the invention. As shown in FIG. 1, a laser diode emits a light beam from a point 2002 located near an end of an active layer 2001 of the laser diode. A far field image of the light beam has a diverging angle .theta..sub.X along X direction in parallel to the active layer 2001 narrower than a diverging angle .theta..sub.Y along Y direction perpendicular thereto. FIGS. 2A and 2B show optical intensity distribution of a light beam (of diameter .phi. of 4 mm) emitted by the laser diode and a light beam going out from a prior art hologram lens along X direction and along Y direction, respectively. The optical intensity distribution 2003 of the light beam emitted by the laser diode are different as described above.
As shown as hatched portions in FIGS. 2A and 2B, if the light beam is incident on the hologram lens, the optical intensity of the outgoing light becomes higher at the outer side than at the inner side. On the other hand, if the light beam is incident on the above-mentioned hologram lens which diffracts a part of the incident light, the optical intensity at the outer side becomes higher than that of the inner side. Because the diverging angle is wide in Y direction, the optical intensity at the outer side becomes higher especially in Y direction.
Next, an effect of side lobe of a light beam is explained. FIGS. 3A and 3B show results of calculation of optical intensity of diverging spot on a thin information medium in the X-direction and in Y direction when a hologram lens which diffracts a part of the incident light is used, wherein the maximum of a main lobe 380 is normalized as 100. The main lobe 380 is a necessary light for recording and reproduction, whereas a side lobe 381 is an unnecessary light which may deteriorate recording pit patterns or reproduced signals. The light intensity of the side lobe 381 is as low as about 1% in the X-direction, while it is about 4% in Y direction or it is somewhat high.
If the light intensity of the side lobe is about 4%, information can be read sufficiently. However, in order to read information stably against disturbances such as vibrations and temperature, it is desirable to decrease the lower lobe to decrease disadvantageous effects of the side lobe.